FoiicaulVs Pendulum Experiment 203 



scribed from its centre perfectly one and the same arc in the same 

 plane' ; but he does not pursue the subject as being foreign to his 

 immediate object.''* It appears also, (Comptes Rendus, 1851, No. 

 65) that ill 1837, Poisson had hinted at such an eflect^ but suppo- 

 sed it of insensible amount. 



Different modes of varying the experiment have been proposed. 

 The following is suggested by Poinsot. If two balls are suspend- 

 ed in contact, with a spring between them, and then by a sudden 

 action of the spring are thrown apart and thus held by some at- 

 tached contrivance, they will retain, after the change of position, 

 their original rate of velocity, or that of the earth where they 

 originally were, and consequently will commence to revolve. It 

 would seem that if the cause mentioned be the source of motion 

 they should not revolve if thrown apart east and west, or at right 

 angles to a meridian. But if viewed in a different light it ap- 

 pears to be a fact that they would revolve in whatever direction 

 separated. Suppose the halves of a sphere to be thus thrown apart 

 and so retained ; they have the momentum of the earth as before, 

 but no relative motion, and consequently they are in the condition 

 of the opposite points of rest of a vibrating pendulum, and will 

 have an appareiJt revolution around the centre between them. 



M. Baudrimont has observed that if torsion could be wholly 

 avoided, a ball suspended or a horizontal bar would exhibit the 

 rotation like a vibrating pendulum. But in these projects there 

 IS a fatal difficulty besides that from torsion. Such bodies, at the 

 time when suspended, have the actual motion of objects at the 

 place where they are, and will retain it; and consequently under 

 no circumstances can they give results like those of the pendu- 

 lum, or show any corresponding effect whatever. 



In performing the pendulum experiment, the ball of the pen- 

 dulum should be an accurately turned sphere or cylinder, having 

 a place of suspension above and an elongated point below, well 

 centered. It is best suspended by means of wire. The simplest 

 mode to fasten it is to solder the wire into a hole drilled of the 

 same size in a larger piece of metal, and secure this last in the 

 ceiling or other support above. The swinging of the pendulum, 

 py flexing the wire at the junction above, will after a while break 

 It off. But if of steel instead of iron, the flexion will be distri- 

 hiited along a portion of its length, and the wire will last longer 

 Without breaking. To mark the motion, a circle (three feet or 

 niore in diameter) divided into degrees, is placed below the pen- 

 dulum; the apparent motion of the plane of vibration, is obser- 

 ved (in north latitude) to be uniformly from left to right, or with 

 the hands of a watch. For starting the pendulum when there is 

 ^o better contrivance, it may be held, previous to letting it swing, 



♦ Pha. Mag. [4] i. 561 



